1. Field of the Invention
This invention relates to the purification of liquids and more specifically to the removal of organic material from water.
2. Description of the Related Art
Biological film treatment processes or bio-reactors are used for effective treatment of organic waste materials. These waste materials include a variety of commercial and industrial discharges, from sanitary wastes to oil and petrochemical spills.
Bio-reactors can be used in a variety of applications, including water purification in fish farms, treatment of water used in food treatment plants, and industrial clean-up water. By ponding such clean-up or waste water mixture, a certain amount of oxidation can be achieved and the chemical oxygen demand (COD) and biological oxygen demand (BOD) of waste water entering the bio-reactor can be reduced. If the necessary nutrients are not already present in the waste water mixture, these nutrients can be added during the ponding phase.
Additionally, storm water regulations now require that contaminated storm water be treated to meet standards before being discarded to recovering streams. Collecting the contaminated storm water and storing it in a pond or lagoon before treatment is recommended so that smaller bio-reactors can be used, thereby reducing the cost, and so that bacteria can be kept active for longer periods. Once treated, ponded water can be used for many commercial purposes such as cooling towers, plant cleanings and process water. By reuse of water, nutrients added to enhance bacterial growth are not lost.
The solids recovered from particulate removal in chicken processing plants can be used as ingredients in chicken feed. Solids not good enough for a feed can be used as fertilizer or otherwise discarded without environmental harm. If oil or fats are present in the waste fluid, a lipophilic surface provides the best contact between bacteria and the oil or fat.
The key component in the bio-reactor is the reactor element. The reactor element must have a large surface in the flow region in order to provide a contact area where microorganisms can live and act on the contaminated material. Two general bio-reactor types have been employed to provide large surface areas in the flow region.
The first type includes the porous diffusion type wherein the discharge solution flows through a porous surface into the interior of a reactor element (as shown and further described in FIG. 3). A porous material, such as sintered diatomaceous earth, provides a tremendously large surface area compared to the volume of material. Using these materials allows construction of reactor elements having very large surface areas. However, while having tremendous surface for bacterial colonies, porous diffusion reactors are, because of the diffusion process, very slow. Waste material, nutrients, and oxygen must all be transported in a liquid solution in order to diffuse into the porous body. Then the processed material and by-products must diffuse outward to the main fluid stream. Additionally, since oxygen, the critical element in aerobic processes, is only slightly soluble in water, a large quantity of water is required to deliver oxygen to the bacteria. For example, at 30.degree. C. water saturated with oxygen can only hold 7.8 lb of oxygen for every 1,000,000 lbs of water. These factors cause diffusion to be a very slow process.
The second type of bio-reactor, commonly referred to as a "bio-deck", uses corrugated sheets of PVC or similar material cemented together at 60.degree. angles, thereby providing a lattice-like structure. Using a bio-deck avoids the problem of low oxygen delivery, since oxygen can be delivered in gaseous form directly to the lattice. Additionally, the slow process of diffusion is avoided by direct wetting of the surface with a contaminant-nutrient solution. However, shortcomings remain with the bio-deck. The structure not only provides less surface than sintered ceramic, but also the "bio-deck" material becomes fouled by bacteria bodies deposited near the intersections of the corrugations. This fouling is caused by the tendency of the flow to take the path of least resistance. At each lattice intersection, disturbances cause slight differences in resistance to flow. The reduced flow, where resistance occurs in any passage, allows dead bacteria to accumulate which further reduces flow. As a result, bio-decks accumulate fouling mass which may be as much as 5-10 times the weight of the original obstruction. This accumulated mass of dead or dying bacteria makes it necessary to shut down the reactor and clean the media to restore operation. Media cleaning is a costly labor-intensive process and incurs additional cost due to equipment being out of service.
Counter-current flow, e.g. liquid flowing downward and air flowing upward, is a popular design for packed tower bio-reactors, but this design presents certain problems. A counter-current reactor typically requires a blower to move the air against the water flow. Secondly, the air will seek the path of least resistance so air will go to paths that have the least water flow, thus defeating the purpose of the tower as previously discussed.
Additionally, when the contamination level is low, as is the situation for fish farm waste water and many types of commercial clean-up water, counter flow systems generally provide an excess quantity of oxygen. For example, a cubic foot of contaminated water with a BOD of 20 ppm would require 0.556 gram of oxygen to reduce the BOD to zero. One cubic foot of air contains 7.33 grams of oxygen or 13 times more oxygen than needed for an equal volume of contaminated water. Despite the excess available oxygen, parts of the bio-deck still lack of sufficient oxygen due to flow patterns which fail to distribute air to all bacterial surfaces.
In any of these systems, the requirements for an effective system are the same. The largest practical surface must be provided which (a) allows microorganisms to accumulate, (b) remains otherwise free of accretion, and (c) has a steady source of both discharge fluid and air.
The typical sequence of purification first requires the removal of particulates. After the particulates have been removed, dissolved material, such as carbohydrates, hydrocarbons, and ammonia, must be converted to non-toxic chemicals, such as carbon dioxide and nitrates or nitrogen. This conversion of dissolved material is generally accomplished by bacteria attached to the surface of tower packing which fills the bio-reactor. The bacterial action produces carbon dioxide and consumes oxygen. The bacteria require a supply of water containing the contaminate. The concentration of contaminants is generally very low, being typically only 1-20 parts per million. In such a case, where one million pounds of water per day may be required to supply the bacteria with one pound of ammonia, it is necessary to have a large flow of water. As an example, a flow of 83.36 gallons per minute for twenty-four hours per day brings one million pounds of water to the bacteria living on the surface of the packing media. Each pound of ammonia (NH.sub.3) oxidized to nitrate (NO.sub.3) requires five pounds of oxygen in the process. This oxygen requirement means that a large flow of air must also be provided to the bacteria.